I. Field of the Invention
This invention relates generally to cardiopulmonary performance analyzing systems, and more particularly to a method for automatically and dynamically calculating a proportionality constant for a flow meter used in the system so as to adjust for changes in ambient conditions, including barometric pressure, temperature and relative humidity, all of which effect the density of the respiratory gases being analyzed.
II. Discussion of the Prior Art
In the Anderson et al. U.S. Pat. No. 4,463,764, there is described a cardiopulmonary exercise system including a plurality of gas sensors connected to a sample line for measuring the percentage concentration of discrete gases in an air mixture being breathed. It also incorporates a flow meter having a pneumotach patient mouthpiece coupled by tubing to a differential pressure sensor for measuring respiratory flow, both inspiratory and expiratory. The gas sensors and the pressure sensor each provide an analog output to a microprocessor-based waveform analyzer. The microprocessor is programmed to process the sensor-derived information for providing a variety of cardiopulmonary performance parameters used by physicians for evaluating the physiologic condition of the patient.
As those skilled in the art appreciate, the flow readings obtained from the differential pressure sensor are subject to variations due to changes in ambient conditions, including barometric pressure, room temperature and relative humidity of the air in the room where testing is underway. Thus, even if the system is accurately calibrated at the factory, using a precision syringe to flow a known volume of air through the flow meter mouthpiece at a known rate and under carefully controllable temperature and relative humidity conditions, the resulting calibration factor, K, used to make the equation: EQU Calibrated Flow=K* Uncalibrated Flow
true, is known to vary with changes in ambient conditions. In the past, K has been empirically determined in the field using a syringe. This new technique eliminates the requirement of using a syringe in determining the proportionality factor, K. It is, accordingly, a principal object of this invention to provide a method for accurately adjusting such calibration factor at any time without a syringe and which takes into account differences in ambient conditions prevailing at the site of the test on the patient from those that may have existed at the factory when the system was originally calibrated.